The present invention relates to an apparatus and method for drilling a pilot bore hole along an underground arcuate path between two surface locations, and then enlarging the pilot bore hole for installing a conduit therein, such as pipe, casing, cable, or a pipeline.
Heretofore, directional drilling apparatus has been provided for drilling an underground pilot bore hole along an arcuate path between two surface locations, and then enlarging the pilot bore hole for receiving a product conduit, such as a pipeline therein. A common apparatus and method has utilized a drill pipe string of relatively small diameter less than two (2) inches with a drill bit thereon forming a relatively small diameter pilot bore hole, which is followed by a relatively large diameter washover pipe such as around five (5) inches in diameter or more, in concentric relation to the drill string and having cutting blades on the leading end thereof for enlarging the initial pilot bore hole. Upon completion of the pilot bore hole as enlarged by the washover pipe, the drill string has been removed and the washover string then connected to a reamer at the exit end for pulling the reamer and connected product conduit along the initial bore hole for reaming to a desired diameter to receive the product conduit.
Thus, the method and apparatus employed heretofore for forming a pilot bore hole normally has included two separate cutting or drilling steps prior to the reaming operation, the first cutting step being the drilling of a small diameter bore hole from a drill bit with a relatively small diameter light weight drill string, and the second cutting step resulting from an enlarging of the bore hole by the washover string in concentric relation to the drill string. Thereafter, the reaming operation has taken place to enlarge the pilot bore hole to receive the product conduit or pipe.
Heretofore, a relatively small diameter drill pipe string of around one and three-fourths (13/4) inches in diameter has been utilized for such pilot bore holes as it was found desirable to have a relatively lightweight drill pipe or string, particularly when drilling under streams where sand and clays are normally encountered with water also being present at times. Under the weight of the drill string, the drill string could sink under certain conditions and it was accepted as common practice in the industry to employ a relatively small diameter drill string in order to keep the weight of the entire drill string at a minimum and to provide flexibility. Such a small diameter drill pipe string thus necessitated the use of a separate washover string in order to provide the necessary strength for the subsequent reaming operation and pulling of the production conduit through the enlarged bore hole.
The utilization of two separate cutting steps or procedures for forming the pilot bore hole prior to the reaming operation results in a costly operation.
Other directional drilling apparatus and methods for underground arcuate bores or openings not employing a washover pipe following the drill string have utilized other types of reamers or cutters which enlarge the initial pilot bore hole by a cutter or reamer moving in the same direction as the initial drill pipe string forming the pilot bore hole.